Electroluminescent materials and devices

ABSTRACT

An improved electroluminescent device has a layer of a first electroluminescent metal complex or organo metallic complex and a layer of a second metal complex or organo metallic complex in which the band gap of the metal in the second electroluminescent metal complex or organo metallic complex is larger than the band gap of the metal in the first electroluminescent metal complex or organo metallic complex.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of the filing date of internationalapplication PCT/GB03/05663 filed Dec. 23, 2003, which claims the benefitof the filing dates of United Kingdom application nos. 0230074.7 filedDec. 24, 2002 and 0230077.0 filed Dec. 24, 2002.

The present invention relates to electroluminescent materials and toelectroluminescent devices.

Materials which emit light when an electric current is passed throughthem are well known and used in a wide range of display applications.Liquid crystal devices and devices which are based on inorganicsemiconductor systems are widely used; however these suffer from thedisadvantages of high energy consumption, high cost of manufacture, lowquantum efficiency and the inability to make flat panel displays.

Organic polymers have been proposed as useful in electroluminescentdevices, but it is not possible to obtain pure colours; they areexpensive to make and have a relatively low efficiency.

Another compound which has been proposed is aluminum quinolate, but thisrequires dopants to be used to obtain a range of colours and has arelatively low efficiency.

Patent application WO98/58037 describes a range of transition metal andlanthanide complexes which can be used in electroluminescent deviceswhich have improved properties and give better results. PatentApplications PCT/GB98/01773, PCT/GB99/03619, PCT/GB99/04030,PCT/GB99/04024, PCT/GB99/04028, PCT/GB00/00268 describeelectroluminescent complexes, structures and devices using rare earthchelates.

U.S. Pat. No. 5,128,587 discloses an electroluminescent device whichconsists of an organometallic complex of rare earth elements of thelanthanide series sandwiched between a transparent electrode of highwork function and a second electrode of low work function with a holeconducting layer interposed between the electroluminescent layer and thetransparent high work function electrode and an electron conductinglayer interposed between the electroluminescent layer and the electroninjecting low work function anode. The hole conducting layer and theelectron conducting layer are required to improve the working and theefficiency of the device. The hole transporting layer serves totransport holes and to block the electrons, thus preventing electronsfrom moving into the electrode without recombining with holes. Therecombination of carriers therefore mainly takes place in the emitterlayer.

It is known that electroluminescent europium organometallic complexesemit light in the red part of the spectrum and application WO98/58037discloses such complexes.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1-16 are formulae drawings for materials which may be used inembodiments of the invention.

FIGS. 17 a and 17 b are diagrammatic sections of an OLED showing layerstructures in embodiments of the invention.

FIGS. 18-20 show the spectrum of the light emitted by devices of theexamples at various applied voltages.

FIGS. 21-25 are performance curves for the exemplified devices.

DETAILED DESCRIPTION OF THE INVENTION

We have now devised an electroluminescent structure which gives improvedred emission.

According to the invention there is provided an electroluminescentdevice which comprises (i) a first electrode, (ii) a layer of a firstelectroluminescent metal complex or organo metallic complex, (iii) alayer of a second metal complex or organo metallic complex and (iv) asecond electrode and in which the band gap of the secondelectroluminescent metal complex or organo metallic complex is largerthan the band gap of the first electroluminescent metal complex ororgano metallic complex.

There can be more than one layer of each of the first and secondelectroluminescent metal complex or organo metallic complexes arrangedalternatively.

In an electroluminescent organometallic complex when an electron dropsfrom one level to a lower level, light is emitted and the larger the gapbetween the levels (the band gap) the higher the energy level differenceand the shorter the wave length of the light emitted. Normally the metalin the first electroluminescent metal complex or organo metallic complexhas a higher HOMO (highest occupied molecular orbital) and a lower LUMO(lowest unoccupied molecular orbital) than the metal in the secondelectroluminescent metal complex or organo metallic complex.

The band gap of the second organometallic complex can be larger than theband gap of the first electroluminescent metal complex or organometallic complex by virtue of the metals and/or organic ligands.

The metal in the first and second electroluminescent metal complex ororgano metallic complex is preferably selected from Sm(III), Eu(II),Eu(III), Tb(III), Dy(III), Yb(III), Lu(III), Gd(III), U(III), U(VI)O₂,Tm(III), Th(IV), Ce(III), Ce(IV), Pr(III), Nd(III), Pm(III), Dy(III),Ho(III), Er(III).

Alternatively the thickness of the layer of the secondelectroluminescent metal complex or organo metallic complex is too thinto emit light, e.g. of less than 10 nanometres.

The metal in the first electroluminescent metal complex or organometallic complex can be any metal which forms an electroluminescentmetal complex or organo metallic complex, provided the band gap is lessthan the band gap of the organometallic complex in the second metalcomplex or organo metallic complex.

Preferred metals in the first electroluminescent metal complex or organometallic complex are europium which emits light in the red region of thespectrum, terbium which emits light in the green region of the spectrumor dysprosium which emits light in the yellow region of the spectrum.

In the present invention the metal in the second electroluminescentmetal complex or organo metallic complex is preferably gadolinium whichemits light predominately in the ultra violet region of the spectrum.This has the advantage being that the ultra violet light emitted has noor a limited effect on the colour of the light emitted by the firstelectroluminescent metal complex or organo metallic complex; anotherpreferred metal is cerium.

The first electroluminescent metal complex or organo metallic complexpreferably has the formula (Lα)_(n)M1 where Lα is an organic complex M1is the metal and n is the valence state of M1.

The second electroluminescent metal complex or organo metallic complexpreferably has the formula (Lα)_(m)M2 where Lα is an organic complex M2is the metal and n is the valence state of M2.

Preferred electroluminescent compounds which can be used in the presentinvention are of formula(Lα)_(x)Mx←Lp  (A)where Mx is the metal, x is the valence state of Mx; Lα and Lp areorganic ligands and Lp is a neutral ligand. The ligands Lα can be thesame or different and there can be a plurality of ligands Lp which canbe the same or different

For example (L₁)(L₂)(L₃)Mx (Lp) where (L₁)(L₂)(L₃) are the same ordifferent organic complexes and (Lp) is a neutral ligand and thedifferent groups (L₁)(L₂)(L₃) may be the same or different.

Lp can be monodentate, bidentate or multidentate and there can be one ormore ligands Lp.

The metal in the organometallic complex forming the first and secondorganometallic layers can be the same provided that the organic ligandis such that the band gap of the organometallic complex forming thesecond layer is larger than the band gap of the organometallic complexforming the first organometallic layer.

Further electroluminescent compounds which can be used in the presentinvention are of general formula (Lα)_(n)MxM3 where M3 is a non rareearth metal, Lα is as herein and n is the combined valence state of Euand M₂. The complex can also comprise one or more neutral ligands Lp sothe complex has the general formula (Lα)_(n)MxM3(Lp), where Lp is asherein. The metal M3 can be any metal which is not a rare earth,transition metal, lanthanide or an actinide. Examples of metals whichcan be used include lithium, sodium, potassium, rubidium, caesium,beryllium, magnesium, calcium, strontium, barium, copper(I), copper(II),silver, gold, zinc, cadmium, boron, aluminium, gallium, indium,germanium, tin(II), tin(IV), antimony(II), antimony(IV), lead(II),lead(IV) and metals of the first, second and third groups of transitionmetals in different valence states, e.g. manganese, iron, ruthenium,osmium, cobalt, nickel, palladium(II), palladium(IV), platinum(II),platinum(IV), cadmium, chromium, titanium, vanadium, zirconium,tantalum, molybdenum, rhodium, iridium, titanium, niobium, scandium,yttrium.

Preferred first electroluminescent metal complex or organo metalliccomplexes are Eu(DBM)₃OPNP which has a band gap of 3.2 eV andEu(TMHD)₃OPNP which has a band gap of 3.7 eV and a preferred gadoliniumcomplex is Gd(DBM)₃Phen, where Phen is phenanthrene, which has a bandgap of 3.8 eV.

For typical terbium complexes the band gap is of the order of 3.7 eV.

In order to increase the conductivity of the second organometalliccomplex layer the layer can be doped with a conductivity improvingadditive such as a powdered metal, conductive polymer.

Other complexes which can be used to form the second electroluminescentlayer are organometallic complexes in which the organic ligand is aboron complex, e.g. of formula

where R₁ and R₂ are the same or different and are hydrogen, andsubstituted and unsubstituted hydrocarbyl groups such as substituted andunsubstituted aliphatic groups, substituted and unsubstituted aromatic,heterocyclic and polycyclic ring structures, fluorocarbons such astrifluoryl methyl groups, halogens such as fluorine or thiophenylgroups; R₁, R₂ and R₃ are the same or different and are hydrogen, andsubstituted and unsubstituted hydrocarbyl groups such as substituted andunsubstituted aliphatic groups, substituted and unsubstituted aromatic,heterocyclic and polycyclic ring structures, fluorocarbons such astrifluoryl methyl groups, halogens such as fluorine or thiophenylgroups; R₁, R₂ and R₃ can also form substituted and unsubstituted fusedaromatic, heterocyclic and polycyclic ring structures, can becopolymerisable with a monomer, e.g. styrene or can be polymer, oligomeror dendrimer substituents.

In place of the terbium other lanthanide, actinide or rare earth metalscan be used.

The invention also provides further red emitters and according to thisaspect of the invention there is provided an electroluminescent devicewhich comprises (i) a first electrode, (ii) a layer of anelectroluminescent europium metal complex or organo metallic complexmixed with an iridium metal complex or organo metallic complex and (iii)a second electrode.

There is preferably also a layer of an electroluminescent europium metalcomplex or organo metallic complex and the invention also provideselectroluminescent devices of structues:—

-   A. (i) a first electrode (ii) a layer of an electroluminescent    europium metal complex or organo metallic complex (iii) a layer of    an electroluminescent europium metal complex or organo metallic    complex mixed with an iridium metal complex or organo metallic    complex and (iv) a second electrode.-   B. (i) a first electrode, (ii) a layer of an electroluminescent    europium metal complex or organo metallic complex mixed with an    iridium metal complex or organo metallic complex, (iii) a layer of    an electroluminescent europium metal complex or organo metallic    complex and (iv) a second electrode.-   C. (i) a first electrode, (ii) a layer of an electroluminescent    europium metal complex or organo metallic complex, (iii) a layer of    an electroluminescent europium metal complex or organo metallic    complex mixed with an iridium metal complex or organo metallic    complex, (iv) a layer of an electroluminescent europium metal    complex or organo metallic complex and (v) a second electrode.

The electroluminescent europium metal complex or organo metallic complexpreferably has the formula (Lα)₃Eu where Lα is an organic complex.

Preferred electroluminescent compounds which can be used in the presentinvention are of formula(Lα)₃Eu Lp  (AI)where Lα and Lp are organic ligands and Lp is a neutral ligand. Theligands Lα can be the same or different and there can be a plurality ofligands Lp which can be the same or different.

For example (L₁)(L₂)(L₃)Eu (Lp) where (L₁)(L₂)(L₃) are the same ordifferent organic complexes and (Lp) is a neutral ligand and thedifferent groups (L₁)(L₂)(L₃) may be the same or different, Lp can bemonodentate, bidentate or multidentate and there ran be one or moreligands Lp.

Further electroluminescent compounds which can be used in the presentinvention are of general formula (Lα)_(n)EuM₂ where M₂ is a non rareearth metal, Lα is a as herein and n is the combined valence state of Euand M₂. The complex can also comprise one or more neutral ligands Lp sothe complex has the general formula (Lα)_(n) Eu M₂ (Lp), where Lp is asherein. The metal M₂ can be any metal which is not a rare earth.Lanthanide or an actinide examples of metals which can be used includelithium, sodium, potassium, rubidium, caesium, beryllium, magnesium,calcium, strontium, barium, boron, copper(I), copper(II), silver, gold,zinc, cadmium, boron, aluminium, gallium, indium, germanium, tin(II),tin(IV), antimony(II), antimony(IV), lead(II), lead(IV) and metals ofthe first, second and third groups of transition metals in differentvalence states, e.g. manganese, iron, ruthenium, osmium, cobalt, nickel,palladium(II), palladium(IV), platinum(II), platinum(IV), cadmium,chromium, titanium, vanadium, zirconium, hafnium, tantulum, molybdenum,rhodium, iridium, titanium, niobium, scandium, yttrium.

A preferred europium complex is Eu(DBM)₃OPNP.

Preferred iridium complexes are iridium acetylacetonate, iridiumdi-acetylacetonate and Ir(dpp)₃

where R₁ and R₂ are as above.

Preferably Lα is selected from β diketones such as those of formulae

where R₁, R₂ and R₃ can be the same or different and are selected fromhydrogen, and substituted and unsubstituted hydrocarbyl groups such assubstituted and unsubstituted aliphatic groups, substituted andunsubstituted aromatic, heterocyclic and polycyclic ring structures,fluorocarbons such as trifluoryl methyl groups, halogens such asfluorine or thiophenyl groups; R₁, R₂ and R₃ can also form substitutedand unsubstituted fused aromatic, heterocyclic and polycyclic ringstructures and can be copolymerisable with a monomer, e.g. styrene. X isSe, S or O, Y can be hydrogen, substituted or unsubstituted hydrocarbylgroups, such as substituted and unsubstituted aromatic, heterocyclic andpolycyclic ring structures, fluorine, fluorocarbons such as trifluorylmethyl groups, halogens such as fluorine or thiophenyl groups ornitrile.

Examples of R₁ and/or R₂ and/or R₃ include aliphatic, aromatic andheterocyclic alkoxy, aryloxy and carboxy groups, substituted andsubstituted phenyl, fluorophenyl, biphenyl, phenanthrene, anthracene,naphthyl and fluorene groups alkyl groups such as t-butyl, heterocyclicgroups such as carbazole.

Some of the different groups Lα may also be the same or differentcharged groups such as carboxylate groups so that the group L₁ can be asdefined above and the groups L₂, L₃ . . . can be charged groups such as

where R is R₁ as defined above or the groups L₁, L₂ can be as definedabove and L₃ . . . etc. are other charged groups.

R₁, R₂ and R₃ can also be

where X is O, S, Se or NH.

A preferred moiety R₁ is trifluoromethyl CF₃ and examples of suchdiketones are, banzoyltrifluoroacetone, p-chlorobenzoyltrifluoroacetone,p-bromotrifluoroacetone, p-phenyltrifluoroacetone,1-naphthoyltrifluoroacetone, 2-naphthoyltrifluoroacetone,2-phenanthoyltrifluoroacetone, 3-phenanthoyltrifluoroacetone,9-anthroyltrifluoroacetonetrifluoroacetone, cinnamoyltrifluoroacetone,and 2-thenoyltrifluoroacetone.

The different groups Lα may be the same or different ligands of formulae

where X is O, S, or Se and R₁ R₂ and R₃ are as above.

The different groups Lα may be the same or different quinolatederivatives such as

where R is hydrocarbyl, aliphatic, aromatic or heterocyclic carboxy,aryloxy, hydroxy or alkoxy, e.g. the 8 hydroxy quinolate derivatives or

where R, R₁, and R₂ are as above or are H or F, e.g. R₁ and R₂ are alkylor alkoxy groups

As stated above the different groups Lα may also be the same ordifferent carboxylate groups e.g.

where R₅ is a substituted or unsubstituted aromatic, polycyclic orheterocyclic ring a polypyridyl group, R₅ can also be a 2-ethyl hexylgroup so L_(n) is 2-ethylhexanoate or R₅ can be a chair structure sothat L_(n) is 2-acetyl cyclohexanoate or Lα can be

where R is as above, e.g. alkyl, allenyl, amino or a fused ring such asa cyclic or polycyclic ring.

The different groups Lα may also be

where R, R₁ and R₂ are as above or

The groups Lp in the formula (A) above can be selected from

where each Ph which can be the same or different and can be a phenyl(OPNP) or a substituted phenyl group, other substituted or unsubstitutedaromatic group, a substituted or unsubstituted heterocyclic orpolycyclic group, a substituted or unsubstituted fused aromatic groupsuch as a naphthyl, anthracene, phenanthrene or pyrene group. Thesubstituents can be, for example, an alkyl, aralkyl, alkoxy, aromatic,heterocyclic, polycyclic group, halogen such as fluorine, cyano, amino,substituted amino etc. Examples are given in FIGS. 1 and 2 of thedrawings where R, R₁, R₂, R₃ and R₄ can be the same or different and areselected from hydrogen, hydrocarbyl groups, substituted andunsubstituted aromatic, heterocyclic and polycyclic ring structures,fluorocarbons such as trifluoryl methyl groups, halogens such asfluorine or thiophenyl groups; R, R₁, R₂, R₃ and R₄ can also formsubstituted and unsubstituted fused aromatic, heterocyclic andpolycyclic ring structures and can be copolymerisable with a monomer,e.g. styrene. R, R₁, R₂, R₃ and R₄ can also be unsaturated alkylenegroups such as vinyl groups or groups—C—CH₂═CH₂—Rwhere R is as above.

L_(p) can also be compounds of formulae

where R₁, R₂ and R₃ are as referred to above, for example bathophenshown in FIG. 3 of the drawings in which R is as above or

where R₁, R₂ and R₃ are as referred to above.

L_(p) can also be

where Ph is as above.

Other examples of L_(p) chelates are as shown in FIG. 4 and fluorene andfluorene derivatives, e.g. as shown in FIG. 5 and compounds of formulaeas shown in FIGS. 6 to 8.

Specific examples of Lα and Lp are tripyridyl and TMHD, and TMHDcomplexes, α, α′, α″ tripyridyl, crown ethers, cyclans, cryptansphthalocyanans, porphoryins ethylene diamine tetramine (EDTA), DCTA,DTPA and TTHA, where TMHD is 2,2,6,6-tetramethyl-3,5-heptanedionato andOPNP is diphenylphosphonimide triphenyl phosphorane. The formulae of thepolyamines are shown in FIG. 11.

The first electrode can function as the cathode and the second electrodecan function as the anode and preferably there is a layer of a holetransporting material between the anode and the layer of theelectroluminescent compound.

The hole transporting material can be any of the hole transportingmaterials used in electroluminescent devices.

The hole transporting material can be an amine complex such as poly(vinylcarbazole), N,N′diphenyl-N,N′-bis (3-methylphenyl)-1,1′-biphenyl4,4′-diamine (TPD), an unsubstituted or substituted polymer of an aminosubstituted aromatic compound, a polyaniline, substituted polyanilines,polythiophenes, substituted polythiophenes, polysilanes etc. Examples ofpolyanilines are polymers of

where R is in the ordio—or meta-position and is hydrogen, C1-18 alkyl,C1-6 alkoxy, amino, chloro, bromo, hydroxy or the group

where R is alky or aryl and R′ is hydrogen, C1-6 alkyl or aryl with atleast one other monomer of formula I above, or the hole transportingmaterial can be a polyaniline; polyanilines which can be used in thepresent invention have the general formula

where p is from 1 to 10 and n is from 1 to 20, R is as defined above andX is an anion, preferably selected from Cl, Br, SO₄, BF₄, PF₆, H₂PO₃,H₂PO₄, arylsulphonate, arenedicarboxylate, polystrenesulphonate,polyacrylate alkysulphonate, vinylsulphonate, vinylbenzene sulphonate,cellulose sulphonate, camphor sulphonates, cellulose sulphate or aperfluorinated polyanion.

Examples of arylsulphonates are p-toluenesulphonate, benzenesulphonate,9,10-anthraquinone-sulphonate and anthracenesulphonate. An example of anarenedicarboxylate is phthalate and an example of arenecarboxylate isbenzoate.

We have found that protonated polymers of the unsubstituted orsubstituted polymer of an amino substituted aromatic compound such as apolyaniline are difficult to evaporate or cannot be evaporated, howeverwe have surprisingly found that if the unsubstituted or substitutedpolymer of an amino substituted aromatic compound is deprotonated thenit can be easily evaporated, i.e. the polymer is evaporable.

Preferably evaporable deprotonated polymers of unsubstituted orsubstituted polymer of an amino substituted aromatic compound are used.The de-protonated unsubstituted or substituted polymer of an aminosubstituted aromatic compound can be formed by deprotonating the polymerby treatment with an alkali such as ammonium hydroxide or an alkalimetal hydroxide such as sodium hydroxide or potassium hydroxide.

The degree of protonation can be controlled by forming a protonatedpolyaniline and de-protonating. Methods of preparing polyanilines aredescribed in the article by A. G. MacDiarmid and A. F. Epstein, FaradayDiscussions, Chem Soc. 88 P319 1989.

The conductivity of the polyaniline is dependant on the degree ofprotonation with the maximum conductivity being when the degree ofprotonation is between 40 and 60%, e.g. about 50%.

Preferably the polymer is substantially fully deprotonated.

A polyaniline can be formed of octamer units i.e. p is four e.g.

The polyanilines can have conductivities of the order of 1×10⁻¹ Siemencm⁻¹ or higher.

The aromatic rings can be unsubstituted or substituted, e.g. by a C1 to20 alkyl group such as ethyl.

The polyaniline can be a copolymer of aniline and preferred copolymersare the copolymers of aniline with o-anisidine, m-sulphanilic acid oro-aminophenol, or o-toluidine with o-aminophenol, o-ethylaniline,o-phenylene diamine or with amino anthracenes.

Other polymers of an amino substituted aromatic compound which can beused include substituted or unsubstituted polyaminonapthalenes,polyaminoanthracenes, polyaminopbenanthrenes, etc. and polymers of anyother condensed polyaromatic compound. Polyaminoanthracenes and methodsof making them are disclosed in U.S. Pat. No. 6,153,726. The aromaticrings can be unsubstituted or substituted, e.g. by a group R as definedabove.

Other hole transporting materials are conjugated polymer and theconjugated polymers which can be used ran be any of the conjugatedpolymers disclosed or referred to in U.S. Pat. No. 5,807,627,PCT/WO90/13148 and PCT/WO92/03490.

The preferred conjugated polymers are poly (p-phenylenevinylene)-PPV andcopolymers including PPV. Other preferred polymers are poly(2,5dialkoxyphenylene vinylene) such as poly(2-methoxy-5-2-methoxypentyloxy-1,4-phenylene vinylene),poly(2-methoxypentyloxy)-1,4-phenylenevinylene),poly(2-methoxy-5-(2-dodecyloxy-1,4-phenylenevinylene) and other poly(2,5dialkoxyphenylenevinylenes) with at least one of the alkoxy groups beinga long chain solubilising alkoxy group, poly fluorenes andoligofluorenes, polyphenylenes and oligophenylenes, polyanthracenes andoligo anthracenes, ploythiophenes and oligothiophenes.

In PPV the phenylene ring may optionally carry one or more substituents,e.g. each independently selected from alkyl, preferably methyl, alkoxy,preferably methoxy or ethoxy.

Any poly(arylenevinylene) including substituted derivatives thereof canbe used and the phenylene ring in poly(p-phenylenevinylene) may bereplaced by a fused ring system such as anthracene or naphthlyene ringand the number of vinylene groups in each polyphenylenevinylene moietycan be increased, e.g. up to 7 or higher.

The conjugated polymers can be made by the methods disclosed in U.S.Pat. No. 5,807,627, PCT/WO90/13148 and PCT/WO92/03490.

The thickness of the hole transporting layer is preferably 20 nm to 200nm.

The polymers of an amino substituted aromatic compound such aspolyanilines referred to above can also be used as buffer layers with orin conjunction with other hole transporting materials.

The structural formulae of some other hole transporting materials areshown in FIGS. 12 to 16 of the drawings, where R₁, R₂ and R₃ can be thesame or different and are selected from hydrogen, and substituted andunsubstituted hydrocarbyl groups such as substituted and unsubstitutedaliphatic groups, substituted and unsubstituted aromatic, heterocyclicand polycyclic ring structures, fluorocarbons such as trifluoryl methylgroups, halogens such as fluorine or thiophenyl groups; R₁, R₂ and R₃can also form substituted and unsubstituted fused aromatic, heterocyclicand polycyclic ring structures and can be copolymerisable with amonomer, e.g. styrene. X is Se, S or O, Y can be hydrogen, substitutedor unsubstituted hydrocarbyl groups, such as substituted andunsubstituted aromatic, heterocyclic and polycyclic ring structures,fluorine, fluorocarbons such as trifluoryl methyl groups, halogens suchas fluorine or thiophenyl groups or nitrile.

Examples of R₁ and/or R₂ and/or R₃ include aliphatic, aromatic andheterocyclic alkoxy, aryloxy and carboxy groups, substituted andsubstituted phenyl, fluorophenyl, biphenyl, phenanthrene, anthracene,naphthyl and fluorene groups alkyl groups such as t-butyl, heterocyclicgroups such as carbazole.

Optionally there is a layer of an electron injecting material betweenthe cathode and the electroluminescent material layer. The electroninjecting material is a material which will transport electrons when anelectric current is passed through electron injecting materials includea metal complex such as a metal quinolate, e.g. an aluminium quinolate,lithium quinolate, Mx(DBM)_(n) where Mx is a metal and DBM is dibenzoylmethane and n is the valency of Mx, e.g Mx is aluminium or chromium. Theelectron injecting material can also be a cyano anthracene such as 9,10dicyano anthracene, cyano substituted aromatic compounds,tetracyanoquinidodimethane a polystyrene sulphonate or a compound withthe structural formulae shown in FIG. 9 or 10 of the drawings in whichthe phenyl rings can be substituted with substituents R as definedabove. Instead of being a separate layer the electron injecting materialcan be mixed with the electroluminescent material and co-deposited withit.

Optionally the hole transporting material can be mixed with theelectroluminescent material and co-deposited with it.

The hole transporting materials, the electroluminescent material and theelectron injecting materials can be mixed together to form one layer,which simplifies the construction.

The anode is preferably a transparent substrate such as a conductiveglass or plastic material which acts as the anode; preferred substratesare conductive glasses such as indium tin oxide coated glass, but anyglass which is conductive or has a conductive layer such as a metal orconductive polymer can be used. Conductive polymers and conductivepolymer coated glass or plastics materials can also be used as thesubstrate.

The cathode is preferably a low work function metal e.g. aluminium,calcium, lithium, magnesium and alloys thereof such as silver/magnesiumalloys, rare earth metal alloys etc; aluminium is a preferred metal. Ametal fluoride such as an alkali metal, rare earth metal or their alloyscan be used as the second electrode, for example by having a metalfluoride layer formed on a metal.

The invention is illustrated in the examples.

EXAMPLE 1

An example of an electroluminescent device according to the invention isshown in FIGS. 17 a, 17 b, of the drawings. A pre-etched ITO coatedglass piece (10×10 cm²) was used. The device was fabricated bysequentially forming layers on the ITO, by vacuum evaporation using aSolciet Machine, ULVAC Ltd. Chigacki, Japan; the active area of eachpixel was 3 mm by 3 mm.

In FIG. 17 a on the ITO coated glass anode (1) there are layers in which(2) is a hole transporting layer of TPD, (3) is a layer ofEu(DBM)₃OPNP(R1), (4) is a layer of Gd(tmhd)₃Phen, (5) is an electrontransmitting layer of aluminium quinolate, (6) is a lithium fluoridelayer and (7) is an aluminium cathode.

When an electric current is passed through the device red light isemitted via (1).

Various structures were formed and the colour coordinates x:y and theirpeak efficiencies measured and the results shown in Table 1. The colourcoordinates are as on the CIE 1931 Chart.

TABLE 1 Reference Cd/m² Cd/A x y 1 2.4 1.13 0.66 0.33 2 15.4 2.01 0.660.33 3 0.9 3.13 0.66 0.33where 1 is—

-   ITO/TPD(35.5 nm)/R1 (23.6 nm)/Gd(tmhd)₃Phen(20.3 nm)/R1(24.2    nm)Alq3(15.5 nm/Al    2 is—-   ITO/TPD(33 nm)/R1(23 nm)/Gd(tmhd)₃Phen(10 nm)R1(10    mm)/Gd(tmhd)₃Phen(10 nm)/R1(23 nm)Alq3(9 nm/Al    3 is—-   ITO/DFDAA(13 nm)/TPD(33 nm)/R1(23 nm)/Gd(tmhd)₃Phen(10 nm)/R1(10    nm)/Gd(tmhd)₃Phen(10 nm)/R1(23 nm)Alq3(9 nm)/Al    where R1 is Eu(DBM)₃OPNP and DFDAA is a buffer layer.

EXAMPLE 2

An electroluminescent device shown in FIG. 17 a was formed. As inexample 1 there are layers 1 to 7 where (1) is ITO, (2) is CuPc (3) isα-NPB (4) is the electroluminescent mixture (5) is Eu(DBM)₃OPNP (6) isAlq₃ (7) is Al to form:—

-   ITO/CuPc(8 nm)/α-NPB(40 nm)/R1(10 nm)/CBP+Ir(dpp)₃(6%)+R1(40%)(20    nm)/R1(20 nm)/BCP(6 nm)/Alq3(20 nm)/Al    where R1 is Eu(DBM)₃OPNP and Ir(dpp)₃ is

An electric current was passed through the device and the properties ofthe emitted light measured and the results are shown in the table 2 andin FIGS. 18 and 21 to 25 of the drawings as configuration 1.

EXAMPLE 3

A device was constructed as in example 1 which had the structure asshown in FIG. 17 b in which (1) is ITO, (2) is CuPc, (3) is α-NPB, (4)is the electroluminescent mixture, (5) is Eu(DBM)₃OPNP, (6) is BCP, (7)is Alq₃ (8) is aluminium. The structure was:—

-   ITO/CuPc(8 nm)/α-NPB(40 nm)/Ir(diacac)₂(dpp)₂(6%)+CBP+R1(40%)(20    nm)/R1(20 nm)/BCP(6 nm)/Alq3(20 nm)/Al    where Ir(diacac)₃ is iridium di-acetylacetonate and CBP is shown in    FIG. 4 b with R being H and is a host compound.

An electric current was passed through the device and the properties ofthe emitted light measured and the results are shown in Table 2 and inFIGS. 19 and 21 to 25 of the drawings as configuration 2.

EXAMPLE 4

A device was constructed as in example 1 which had the structure of FIG.17 a and consisted of

-   ITO/CuPc(8 nm)/α-NPB(40 nm)/R1(40%)+Ir(acac)₃(6%)+CBP(20 nm)/BCP(6    nm)/Alq3(20 nm)/Al    where Ir(acac)₃ is iridium acetylacetonate and BCP is bathocupron.

An electric current was passed through the device and the properties ofthe emitted light measured and the results are shown in Table 2 and inFIGS. 20 to 25 of the drawings as configuration 3.

TABLE 2 Best Current Efficiency Ref. cd A⁻¹ x y Example 1 3.0 0.63 0.35Example 2 1.8 0.66 0.33 Example 3 0.8 0.63 0.34

1. An electroluminescent device which comprises: (i) a first electrodewhich functions as an anode; (ii) a second electrode which functions asa cathode; and, (iii) between said first and second electrodes, thefollowing layers (a) to (e) in sequence: (a) a layer of a hole transportmaterial; (b) a first layer comprising a first electroluminescent metalcomplex or a first electroluminescent organometallic complex having aband gap; (c) a layer comprising a second electroluminescent metalcomplex or a second electroluminescent organometallic complex having aband gap, wherein the band gap of the second complex is larger than thatof the first complex and wherein the highest occupied molecular orbital(HOMO) of the first complex is higher, and the lowest unoccupiedmolecular orbital (LUMO) of the first complex is lower, than those ofthe second complex, and wherein the layer of second complex has athickness of about 10 nm or less; (d) a second layer comprising thefirst complex; and, (e) a layer of an electron transport material; and,further wherein the second electroluminescent metal complex or secondelectroluminescent organometallic complex emits light in the ultravioletregion of the spectrum.
 2. The device of claim 1, wherein the firstelectrode/anode is an ITO layer.
 3. The device of claim 1, wherein thehole transport material comprisesN,N′-diphenyl-N,N′-bis-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine(TPD), HTM-1, TPTE, α-NPB or mTADATA.
 4. The device of claim 1, whereinthe first electroluminescent metal complex or first electroluminescentorganometallic complex emits light in the red, green or yellow regionsof the spectrum.
 5. The device of claim 1, wherein the firstelectroluminescent metal complex or first electroluminescentorganometallic complex is a complex including Eu, Tb or Dy.
 6. Thedevice of claim 1, wherein the first electroluminescent complex or firstelectroluminescent organometallic complex is Eu(TMHD)₃OPNP orEu(DBM)₃OPNP.
 7. The device of claim 1, wherein the secondelectroluminescent metal complex or second electroluminescentorganometallic complex is a complex including Gd or Ce.
 8. The device ofclaim 1, wherein the second electroluminescent metal complex or secondelectroluminescent organometallic complex is Gd(DBM)₃Phen wherein Phendesignates the neutral ligand phenanthroline.
 9. The device of claim 1,wherein said second electrode comprises a material selected fromaluminum, calcium, lithium, and silver/magnesium alloys.
 10. The deviceof claim 1, wherein the electron transport layer comprises a metalquinolate.
 11. The device of claim 1, wherein the electron transportmaterial layer comprises aluminum quinolate or lithium quinolate.
 12. Anelectroluminescent device which comprises: (i) a first electrode whichfunctions as an anode; (ii) a second electrode which functions as acathode; and, (iii) between said first and second electrodes, thefollowing layers (a) to (c) in sequence: (a) a layer of a hole transportmaterial; (b) a composite electroluminescent layer comprising insequence alternating sub-layers of a first electroluminescent metalcomplex or first organometallic complex having a band gap and a secondelectroluminescent metal complex or second organometallic complex havinga band gap, the composite layer including at least two sub-layers of thesecond complex and at least three sub-layers of the first complex,wherein the band gap of the second complex is larger than that of thefirst complex and wherein the highest occupied molecular orbital (HOMO)of the first complex is higher, and the lowest unoccupied molecularorbital (LUMO) of the first complex is lower, than those of the secondcomplex, and wherein each layer of the second complex has a thickness ofabout 10 nm or less; and, (c) a layer of an electron transport material;and, further wherein the second electroluminescent metal complex orsecond electroluminescent organometallic complex emits light in theultraviolet region of the spectrum.
 13. The device of claim 12, whereineach of the sub-layers of first or second complex located between thefirst and the last sub-layers of the first complex has a thickness ofabout 10 nm.
 14. An electroluminescent device which comprises: b. afirst electrode which functions as an anode; c. a second electrode whichfunctions as a cathode; and, d. between said first and secondelectrodes, the following layers (a) to (g) in sequence: (a) a layer ofa hole transport material; (b) a first layer comprising a firstelectroluminescent metal complex or a first electroluminescentorganometallic complex having a band gap, such first layer having athickness of about 23 nm; (c) a layer comprising a secondelectroluminescent metal complex or a second electroluminescentorganometallic complex having a band gap, wherein the band gap of thesecond complex is larger than that of the first complex, and wherein thehighest occupied molecular orbital (HOMO) of the first complex ishigher, and the lowest unoccupied molecular orbital (LUMO) of the firstcomplex is lower, than those of the second complex, and wherein thislayer has a thickness of about 10 nm; (d) a second layer comprising thefirst complex, this layer having a thickness of about 10 nm; (e) asecond layer comprising the second complex, this layer having athickness of about 10 nm; (f) a third layer comprising the firstcomplex, this layer having a thickness of about 23 nm, and, (g) a layerof an electron transport material; and, further wherein the secondelectroluminescent metal complex or second electroluminescentorganometallic complex emits light in the ultraviolet region of thespectrum.
 15. The device of claim 14, wherein the first electrode/anodeis an ITO layer.
 16. The device of claim 14, wherein the hole transportmaterial comprisesN,N′-diphenyl-N,N′-bis-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine(TPD), HTM-1, TPTE, α-NPB or mTADATA.
 17. The device of claim 14,wherein the first electroluminescent metal complex or firstelectroluminescent organometallic complex emits light in the red, greenor yellow regions of the spectrum.
 18. The device of claim 14, whereinthe first electroluminescent metal complex or first electroluminescentorganometallic complex is a complex including Eu, Tb or Dy.
 19. Thedevice of claim 14, wherein the first electroluminescent complex orfirst electroluminescent organometallic complex is Eu(TMHD)₃OPNP orEu(DBM)₃OPNP.
 20. The device of claim 14, wherein the secondelectroluminescent metal complex or second electroluminescentorganometallic complex is a complex including Gd or Ce.
 21. The deviceof claim 14, wherein the second electroluminescent metal complex orsecond electroluminescent organometallic complex is Gd(DBM)₃Phen whereinPhen designates the neutral ligand phenanthroline.
 22. The device ofclaim 14, wherein said second electrode comprises a material selectedfrom aluminum, calcium, lithium, and silver/magnesium alloys.
 23. Thedevice of claim 14, wherein the electron transport layer comprises ametal quinolate.
 24. The device of claim 14, wherein the electrontransport material layer comprises aluminum quinolate or lithiumquinolate.